The present invention relates to a computer-implemented method of preoperatively determining the optimized external shape of a prosthetic femoral hip stem (implant) having a proximal and a distal segment as well as an intermediate segment between them, the proximal and distal segments to be press-fitted with an intramedullary canal of the femur of a patient and the intermediate segment having a reduced cross section for injecting cement, for use in partial cementing hip replacement procedures, and of a reamer for reaming a cavity in the femur of a patient for implanting the prosthetic femoral hip stem.
Accidents, bone disease and age cause damage to different human bones and joints, which leads to bones and joints being replaced or amended. All over the world several hundred thousands patients undergo surgeries for joints replacement or bone amendments due to accidents while several other thousands undergo implant operations due to age and bone diseases.
Everyone's bones are unique. They may be similar in size and shape, but the dimensions are different from patient to patient. Cemented implants were able to address these variations with only a few sizes, letting them “customize” the fit to the individual's bone. Without cement, it became much more difficult to address anatomical variations and still provide the required stability and contact. Most manufacturers of the cementless implants simply took a given design and made it larger or smaller to create a range of sizes. Unfortunately all patients are not built that way. The target market of the first custom made, however, traditional custom implants (i.e. in patients with tumors or trauma). If a patient has truly unique hip joint anatomy, doctors prefer the custom approach. In most revision cases they also prefer to use custom implants design. Custom-made implants up until this time were used mainly for tumor or trauma cases where a part is missing or badly misshapen.
The idea of custom-made cementless implants for the femur was developed. In this way a unique implant is made for each patient, thereby addressing the patients' individual anatomic variations. The challenge is to manufacture an implant accurately and without too much additional cost. The tailor-made implants could prove more durability over a wider range of motion for patients.
In presence of marked deformities, custom made prostheses may be the only viable solution. When compared with standard cementless design, custom-made prostheses seem to achieve better contact at the bone-implant interface leading to the least stress shielding.
Unfortunately bone quality for hip replacement patients are not the same (tumor and bone diseases). So, part of the patient bones (high quality) could withstand the cementless technique, while the rest (bad quality) could only withstand the cemented hip technique.
In view of this, hybrid-segmented or (tri-compartmental) hip implants (prosthetic femoral hip stems) were developed. Such a hip implant comprises a proximal and a distal segment and an intermediate segment between them. The proximal and distal segments of the implant are manufactured so as to provide a press-fit with the patients' intramedullary canal. In contrast the intermediate segment is provided with a reduced cross section so as to create a clearance between the implant and the bone. Thereby the implant can be cemented to the patients' bone in the intermediate section.